JPH0151144B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a signal coupling circuit that couples an input signal AC or DC to a subsequent circuit.

[Prior art and its problems]

In signal measuring devices such as oscilloscopes, input signals are selectively switched between AC coupling and DC coupling using a signal coupling circuit. Conventional signal coupling circuits are
It consists of a capacitor and a mechanical switch connected in parallel to the capacitor. When this mechanical switch is turned off (non-conducting), it becomes AC coupling, and when it is turned on (conducting), it becomes DC coupling.
However, other conventional signal coupling circuits use electronic switches because mechanical switches are difficult to control remotely. When a transistor is used as an electronic switch, the collector and emitter of the transistor are connected to both ends of a capacitor, and the base voltage of the transistor is controlled to turn the transistor on and off. However, when the transistor is turned on for DC coupling, the base current flows into the signal path, which has a disadvantage in that it adversely affects the input signal.

[Purpose of the invention]

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a coupling circuit which uses a transistor as a coupling changeover switch and whose input signal is not affected by the base current of the transistor.

[Summary of the invention]

The signal coupling circuit of the present invention includes an amplifier having an inverting input terminal and a non-inverting input terminal and obtaining an output signal from an output terminal, and an amplifier having one end receiving an input signal and the other end connected to the inverting input terminal of the amplifier. A series circuit of an input resistor and a capacitor, a feedback resistor connected between the output terminal and the inverting input terminal of the amplifier, a transistor as a coupling changeover switch whose collector and emitter are connected to both ends of the capacitor, and an inverting amplifier. A compensation circuit connected to the input terminal, and a control circuit that switches the base voltage of the transistor to control on and off of the transistor, and switches the voltage at the non-inverting input terminal of the amplifier in accordance with this on and off control. It is equipped with When the transistor is on, the base current of this transistor flows in the signal path, and the compensation circuit cancels out this base current to compensate for the effect of the base current. Furthermore, when the transistor is off, the base current of the transistor does not flow, so a canceling current of the compensation circuit flows into the signal path, but the influence of this canceling current is compensated by the voltage at the non-inverting input terminal of the amplifier. Therefore, even if a transistor is used as a coupling changeover switch, the effect of the base current of this transistor does not appear on the output signal from the output terminal of the amplifier.

[Embodiments of the invention]

Hereinafter, a preferred embodiment of the present invention will be described with reference to the accompanying drawings. The attached figure is a circuit diagram of a preferred embodiment of the present invention. Input terminal 10 is input resistor 13
It is connected to the inverting input terminal (ie, connection point 41) of an operational amplifier (hereinafter simply referred to as OP amplifier) 36 via a parallel circuit of phase compensation capacitor 15 and AC coupling capacitor 19. A parallel circuit of a feedback resistor 46 and a high frequency compensation capacitor 48 is connected between the output terminal and the inverting input terminal of the OP amplifier 36. The output terminal of the OP amplifier 36 is connected to the output terminal 50
to get the output signal. The low frequency gain of the OP amplifier for the input signal at the input terminal 10 is R ₄₆ /R ₁₃ (where R ₁₃ and R ₄₆ are each connected to the resistor 13
and a value of 46). Connection point 41 is resistor 37
It is connected to a negative voltage source through a resistor 39 and grounded through a resistor 39. A resistor 37 and a compensation circuit are configured.

Switching transistor 18 for coupling switching
The collector and emitter of the capacitor 19 are connected to both ends of the capacitor 19. The control circuit 16 includes a transistor 18
The input signal is applied to the subsequent stage through AC or DC coupling. The base of transistor 18 is connected to a positive voltage source through resistor 20 and through level shifting diodes 22 and 24.
It is connected to connection point 26. Connection point 26 is connected to a negative voltage source through a resistor 30 and connected to a voltage dividing resistor 3
It is grounded via 2 and 34. Also, connection point 26
is grounded via switch 28. The common connection point of resistors 32 and 34 is connected to the non-inverting input of OP amplifier 36. These elements 20 to 34 constitute the control circuit 16.

When switch 28 is closed, connection point 26 and OP
The non-inverting input terminal of amplifier 36 is at ground potential.
Since the OP amplifier 36 has a DC negative feedback path, the potential at the inverting input terminal is the same as the potential at the non-inverting input terminal, and the inverting input terminal of the amplifier 36, that is, the emitter of the transistor 18, also becomes the ground potential. Also, the base of transistor 18 is raised above ground potential by diodes 22 and 24. Therefore, the transistor 18 is turned on and the input becomes DC coupled.
On the other hand, when the switch 28 is opened, the potential at the connection point 26 becomes lower than the potential at the non-inverting input terminal (ie, the potential at the inverting input terminal) of the amplifier 36. transistor 18
The base potential of is higher than the potential at node 26 by the offset of diodes 22 and 24, but lower than the potential at the inverting input of amplifier 36. Then,
Transistor 18 is turned off and the input is AC coupled. If the input is DC coupled, transistor 1
Since the base current of 8 is applied to the inverting input terminal of the OP amplifier 36, the DC level of the output of the OP amplifier 36 tends to fluctuate compared to the case of AC coupling.
The present invention compensates for such variations. This compensation will be explained below.

In the case of DC coupling, since switch 28 is closed, the non-inverting input of OP amplifier 36 is at ground potential. Further, since the OP amplifier 36 has a DC negative feedback path of the resistor 46, the inverting input terminal is also at ground potential.
If the current flowing from the connection point 41 to the compensation circuit of the resistors 37 and 39 is set approximately equal to the base current of the transistor 18, all of this base current flows to the compensation circuit of the resistors 37 and 39 and is canceled out.
It does not flow through the feedback resistor 46. Therefore, the output level of the OP amplifier 36 is not affected by the base current of the transistor 18, and becomes the ground potential when there is no signal.

In the case of AC coupling, since the transistor 18 is turned off, no base current flows, but current flows from the resistor 46 to the compensation circuit of the resistors 37 and 39. However, in this case, since the switch 28 is open, the non-inverting input terminal of the OP amplifier 36 has a slightly negative potential. Due to the effect of negative feedback, the connection point 41 also has a negative potential equal to that of the non-inverting input terminal, and the resistor 37
The current flowing through the circuits 39 and 39 changes. The voltage drop across the resistor 46 due to this current causes the OP amplifier 36 to
It is set to be equal to the absolute value of the voltage at the non-inverting input terminal of . Therefore, the output level of the OP amplifier is the same as in the case of DC coupling, and is at ground potential when there is no signal. Therefore, the output signal at terminal 50 is not affected by the current flowing through the compensation circuit of resistors 37 and 39 that compensated for the base current of transistor 18.

That is, the values of the resistors 37 and 39, the value of the negative voltage source connected to the resistor 37, and the voltage at the non-inverting input terminal of the OP amplifier 36 are appropriately adjusted according to the base current of the transistor 18 and the value of the resistor 46. If selected, it is possible to compensate for drift in the output signal of the OP amplifier 36 due to the base current of the transistor 18 during DC coupling. Note that the compensation circuit of resistors 37 and 39 has one end connected to a negative voltage source.
A real resistor can also be used. Further, although the transistor 18 is of the NPN type in this embodiment, the base current can be similarly compensated for when DC coupling is performed even with a PNP type, if the polarity of the voltage is considered.

〔Effect of the invention〕

As described above, according to the present invention, even if a transistor is used as the coupling changeover switch of the signal coupling circuit, the influence of the base current when the transistor is on can be compensated for by the compensation circuit. Also, the effect of the current in the compensation circuit when the transistor is off can be compensated for by adjusting the voltage at the non-inverting input of the amplifier. Therefore, the transistor can be used as a coupling changeover switch without affecting the signal path of the signal coupling circuit.

[Brief explanation of drawings]

The attached figure is a circuit diagram of a preferred embodiment of the present invention. In the figure, 13 is an input resistor, 16 is a control circuit, 18 is a transistor, 19 is a capacitor, 3
6 is an amplifier, 37 and 39 are compensation circuit resistors,
46 is a feedback resistor.

Claims (1)

[Claims] 1. An amplifier having an inverting input terminal and a non-inverting input terminal and obtaining an output signal from an output terminal, and an input resistor and a capacitor connected in series with one end receiving the input signal and the other end connected to the inverting input terminal of the amplifier. a feedback resistor connected between the output terminal and the inverting input terminal of the amplifier, a transistor having a collector and an emitter connected to both ends of the capacitor, and a compensation circuit connected to the inverting input terminal of the amplifier. , a control circuit that controls on and off of the transistor by switching the base voltage of the transistor, and switches the voltage at the non-inverting input terminal of the amplifier in accordance with the on and off control, and when the transistor is on, A signal coupling characterized in that the influence of the base current of the transistor is compensated by the compensation circuit when the transistor is off, and the influence of the current by the compensation circuit is compensated by the voltage at the non-inverting input terminal of the amplifier when the transistor is off. circuit.